Impedance spectroscopy is a characterization technique used in many technical fields.
As an illustration, impedance spectroscopy is used in the fields of corrosion, for example to estimate the corrosion rate of a metal piece, electrodeposition, or determining a state of health of an electrochemical device such as a fuel cell.
In the latter case, the impedance spectroscopy technique can be used to detect the engorgement and drainage of a fuel cell, as described in the article by Fouquet et al. entitled “Model based PEM fuel cell state-of-health monitoring via ac impedance measurements” and published in 2006 in the Journal of Power Sources, 159, 905-913.
As shown in FIG. 1, a current ie is applied to the cell 1, which has a sequence of sinusoidal perturbations around a polarization point (FIG. 2). The current ie is applied by an active load 2 delivering a direct current on which said perturbations are superimposed. The frequency thereof is controlled by an impedance analysis device 3. The perturbations have a low amplitude and scan a large frequency range.
The voltage in response to these perturbations is measured at the terminals of the cell. The impedance analyzer 3 gives the evolution in a Nyquist plane of the imaginary part of the impedance as a function of its real part.
The perturbations are traditionally applied so as to scan a large frequency range, going from high frequencies to low frequencies, the frequencies being spaced apart logarithmically. The frequency range can go from several millihertz to several tens of kilohertz.
Thus, the high-frequency part is scanned very quickly, while for the low frequencies, the measurement time becomes non-negligible. For example, a second is sufficient to go from 10 kHz to 500 Hz with about one hundred points per decade, while several minutes are necessary for measurements of frequencies below 1 Hz.
The exploitation of the experimental data requires that the cell remain stable for the time it takes to apply said perturbations, i.e. the average value of the polarization current and that of the response voltage remain constant over time.
However, this hypothesis cannot be verified, in particular during deteriorations or temporary failures of the electrical system. As an example, the drainage and engorgement of a PEM-type fuel cell make the cell unstable, which makes it impossible to exploit the impedance measurement.
The current impedance spectroscopy technique has the drawback of not making it possible to directly identify, during scanning of the frequency range, a change in the stability condition of the electrical system.